(OPRAH.com) -- Physical attraction may be as old as time, but new studies are beginning to uncover the science behind sex appeal. Unexpected factors -- like biochemical odors, face shape and voice pitch -- just might have more to do with your choice of mate than anyone ever expected.

Karl Grammer and Elizabeth Oberzaucher are leading the research on the human scent's influence on sexual attraction. They've found that when women are ovulating, they produce copulins, a scent that attracts men.

The researchers believe when a man gets a whiff of copulins, his testosterone levels rise. As a result, he secretes androstenone, an odor that repels women who aren't ovulating.

"Bars and nightclubs across the world are the battlefield for this invisible biochemical war," Oprah Winfrey says.

Sex therapist Dr. Laura Berman says attraction has a lot more to do with science and evolution than people might think.

"We are innately all puppies in heat," she says. "We are capable of discerning 10,000 different scents consciously. But then there's a whole realm of unconscious scents that we're not even aware that we're smelling." Oprah.com: Get Dr. Berman's 5 steps to better sex

These odors let people know when a woman is fertile and when a man's testosterone levels are high, Berman says. As a result, individuals can tell when someone else is most ripe for reproduction. "It's all about survival of the species," she says.

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One common complaint Berman hears among women with relationship problems is that they love their partner, but they're not in love with him.

"There is a difference between love and chemistry," Berman says. "Take a really good whiff of him without cologne when he's relatively clean. If that smell turns you on, that's a really good cue of chemistry." Oprah.com: Can the chemistry come later?

In one recent study, Berman says, researchers had women smell men's T-shirts. The women were most attracted to the shirts of men with a different major histocompatability complex (MHC) from them. MHC is a collection of genes that are related to immune systems.

"We unconsciously want to mate with someone who has a different immune system than ours because that helps with the survival of our offspring," Berman says.

The T-shirt study also found that women taking hormonal contraception were attracted to men who had similar MHC as they did.

"If you're on the pill, your body is being tricked into think you're pregnant, so you're not ovulating," Berman says. "The bad news there is that they have looked at these couples who have similar MHC, and not only do they have higher levels of infidelity and higher levels of marital discord, but they also have higher infertility issues."

Another unexpected factor that has been found to directly affect attraction is voice pitch.

Gordon Gallup, Ph.D., a professor at the State University of New York at Albany, says that women with higher estrogen levels have higher voices, which makes them more desirable to men. "When females are midcycle, when they're the most fertile, the most likely to conceive, their voices are rated as being significantly more attractive," he says.

It's hard to pinpoint what distinguishes a gorgeous face from an average one, but some researchers are getting pretty close. Psychologist Lisa DeBruine, Ph.D., of the University of Aberdeen in Scotland says she's found that women's faces get more attractive to men when they are ovulating. "We're not entirely clear why there's this difference, but we think that the women might look healthier, have a bit of a healthier glow when they are ovulating."

Symmetry is another factor that determines a face's attractiveness. Kendra Schmid, an assistant professor of biostatistics at the University of Nebraska Medical Center, says there is a formula for the "perfect" face. She uses 29 different measurements to determine someone's appeal on a scale of 1 to 10.

To start, Schmid says the ratio of the length of the face to the width of the face should be 1.6, also known as the golden ratio."The face should also be divided into three equal pieces vertically," she says. "The forehead, then [the bottom of the forehead to] the nose and then from the nose to the chin."

Who's face is the most "perfect," according to Schmid's measurements? No big surprise here! "Brad Pitt's is the highest that I've ever used the [formula] on," she says. "He was a 9.3 [out of 10]."

Brad's partner, Angelina Jolie, didn't fare too badly either. "Angelina was a 7.67, and that's pretty high," Schmid says. "Most people rate about 4 to 6. ... The thing that is probably lowering her score is the thing that she's most famous for -- her full lips." Schmid says that ideally the width of a mouth should be twice the height of the lips.

Other celebrities Schmid tested were Halle Berry, who scored a 7.36, and Hugh Jackman, a 6.45. "There's never been anyone who was a perfect 10," Schmid says. "If you're out there, we're looking for you!"

Not everything that goes into finding a partner is biological. Researchers have also found that if a woman looks at the face of a man whom she knows nothing about, she will give it a rating on a scale of 1 to 10 that's different than if she is shown the same face and a corresponding income. When a man makes a lot of money, a woman will rate him higher on an attractiveness scale than she would that same man with a smaller income. Oprah.com: What do women really want?

Berman says this isn't a case of women being gold diggers.

"It goes back again to evolution. When we were having babies who were very dependent on us, we couldn't hunt and take care of ourselves, so we were looking for the man who had the most social status, who was the best hunter, who was going to bring home the biggest chunk of meat for our babies," she says. "It's the same thing today."

Smell, voice pitch and face symmetry might be unconscious detectors of attractiveness, but if you're trying to consciously assess the future of a relationship, Berman says there's one surefire way.

"You really learn a lot from a kiss. ... Sixty-six percent of women say they would dump a guy after a bad first kiss," she says. "It's not only because you're getting that good whiff of him so you're seeing if you like the way he smells. All your senses are engaged, and you get a sense of his sensuality, his connection, whether he's a take-charge kind of guy or more of a stepping back kind."

When it comes to relationships, Berman says kissing frequency is a direct indicator of happiness. "It turns out that couples who kiss and cuddle regularly are eight times less likely to be stressed and depressed than couples who don't get that," she says. "You have to have a minimum of a 10-second kiss every day."